Effects of cortical stimulation on reciprocal inhibition in humans

We attempted to demonstrate convergence onto human spinal Ia inhibitory interneurons from Ia afferents and from fast conducting corticospinal axons. Stimulation of the common peroneal nerve at or below the threshold of the alpha motoneuron axons resulted in inhibition of the soleus H-reflex, attributed to reciprocal inhibition. Magnetic stimulation over the contralateral motor cortex resulted in complex modulations of the soleus H-reflex, including a short latency-inhibition. To test for convergence, the two stimuli were given together so that the two inhibitions coincided. When each stimulus alone produced clear inhibition, the inhibition produced by both stimuli was less than expected, implying an interaction between the two volleys, for example, occlusion occurring in interneurons or motoneurons. When the H-reflex was relatively unaffected by one or other conditioning volley, the inhibition produced by the combined stimulation was greater than expected, as might be expected with convergence onto a common pool of interneurons.